Sheep erythrocytes sensitized with a hybridoma immunoglobulin capable of binding specifically with Type III pneumococcal polysaccharide (SSS-III) were used as indicator cells for the detection of plaque-forming cells (PFC) making anti-idiotypic antibody. Significant numbers of PFC making antibody against the CC4.6 idiotype, a common idiotype found in the serum of mice immunized with SSS-III, were detected during the late stages of an antibody response to an optimally immunogenic dose of SSS-III; larger numbers were found in mice given a high dose (tolerogenic) dose of SSS-III. The relationship of such antibody to the mode of action of thymus-derived suppressor and amplifier cells is being investigated.